The primary aim of this proposal is to extend and explore our recent finding that potentials recorded during the depolarization of cardiac fibrs are strongly influenced by orientation of cardiac fibers. Traditional application of volume conductor theory to cardiac depolarization has been based on the assumption that the primary dipolar generators during depolarization of ventricular muscle are perpendicular to the macroscopic boundary between resting and depolarized muscle. Our studies have indicated that this assumption is largely incorrect. During depolarization the generators are, rather, oriented nearly parallel to the long axes of cardiac fibers at the boundary. Because of this orientation of the dipoles, the fields produced external to the wave of depolarization are markedly different from those which would be predicted by the conventional assumption. Indeed, as we have demonstrated, an approaching wave of depolarization will frequently generate an initial negative potential rather than a positive potential (transverse approach). These findings are essential to an understanding of the electrocardiogram and appear to account for the fact that there have been no satisfactory predictions of electrocardiographic potentials from depolarization pathways. Our present demonstration of the "axial dipole" orientation must first be extended to blocks of tissue which are either anatomically or "physiologically" isolated so that there is activity in only one section of myocardium. A second step will be to extend this finding to blocks of tissue which are depolarized through Purkinje fibers. Finally, we will predict cardiac surface potentials from depolarization pathways. In earlier studies we have accumulated a large amount of reliable data concerning ventricular depolarization patterns. We feel that by combining these data with proper anatomical data concerning the thoracic geometry of the dog, plus fiber orientation within the myocardium, we should be able to ultimately quantitatively predict the shape of cardiac surface, cylinder surface, or torso surface QRS complexes.